i) Field
The present relates to a new dry mixed product having re-dispersible cellulose filaments associated physically with a carrier and the method for producing this dry mixed product. The method of producing the dry mixed product begins with cellulose filaments and their incorporation into/onto a wet carrier, such as wood or other plant pulps. Surprisingly, the wet mixed cellulose filament/pulp product can be dried in conventional drying equipment without the cellulose filaments losing their re-dispersible property.
ii) Description of the Prior Art
There is considerable amount of research and development activities worldwide to isolate and commercialize cellulose-based nano- or quasi-nano suprastructures from wood, plant, marine animals, algae and bacteria sources to improve existing materials or to design and develop a variety of entirely new products in a wide variety of applications and markets as described by Shatkin et al (Tappi Journal, 13(5):9-16 and 13(6):57-69 (2014)). Cellulose nanofilaments (CNF) disclosed by Hua et al (CA 2,799,123), defined herein and referred to as cellulose filaments (CF), have in a preferred embodiment lengths of over 100 μm and a width in submicron range. The CF can be produced by multi-pass high consistency refining of wood or plant fibres such as a bleached softwood kraft pulp as described by Hua et al in US Pat. Application No. 20130017394 incorporated herein by reference. The CF is structurally different from other cellulose fibrils such as microfibrillated cellulose (MFC), nanofibrillated cellulose (NFC), or nanocellulose in that it comprises high-aspect-ratio cellulose fibrils physically detached from each other, and from parent fibres, while MFC or NFC are either fibril bundles or short fibrils, typically less than 1 micrometer. CF exhibits exceptional reinforcement properties due to their high aspect ratio which can exceed 1000, that is much higher than microfibrillated or nanofibrillated cellulose, or cellulose nanofibrils prepared using other mechanical methods (Turbak et al 1983, U.S. Pat. No. 4,374,702; Matsuda et al 2001, U.S. Pat. No. 6,183,596; Choi et al 2010, EP 1 859 082 B1; Laukkanen et al 2013, US Pat. Application No. 2013/0345416 A1). CF is generally made at consistencies greater than 20%, preferably between 30 and 45% fibre suspension with addition of water (US Pat. No. 2013/0017394). Most other methods to produce MFC/NFC are typically carried out in aqueous suspensions at fibre consistencies lower than 10% and preferably in the 1-6% range (Matsuda et al 2001, U.S. Pat. No. 6,183,596; U.S. Pat. No. 6,214,163; Li et al 2012, CN 2012-10282759; Bras et al 2014, WO 2014/001699 A1; Saito et al 2006 Biomacromolecules, 7:1687-1691; 2007 Biomacromolecules, 8:2485-2491; 2009 Biomacromolecules, 10:1992-1996; Da Sil Va Perez et al 2010 TAPPI Nano 2). The resultant final products of MFC/NFC made at low consistency have a gel-like structure (Turbak et al 1983, U.S. Pat. No. 4,374,702) whereas CF made above 20% consistency has a semi-dry wood pulp-like appearance but still contains a substantial amount of residual water after manufacturing.
Ideally, commercial nanocellulosic or quasi-nano cellulosic materials should be transported to end-user's location in a fully dry form in order to reduce shipping cost and to provide long product shelf-life. However, the difficulty of preparing dry products without decreasing their dispersibility in aqueous media represents a serious impediment to their successful commercialization. This drying issue which is shared by all cellulose microfibrils and nanofibrils is generally ascribed to so-called hornification phenomenon which impairs mechanical properties as discussed by Diniz et al (Wood Sc. Tehcnol, 37:489-494, 2004). In the field of wood pulp making, hornification describes changes in fibre morphology after wood pulp fibres have been dried for the first time. Hornification is attributed to many factors which include the formation of irreversible hydrogen bonds (H-bonds) and/or the formation of lactone bridges. Hornification provokes agglomeration of fibrils via self-assembly and therefore represents an obstacle to the recovery of the quasi- or true nanometric dimensions of never-dried cellulose fibrils when these materials are re-mixed in water using conventional low and medium consistency pulpers. Indeed, a dense assembly of dry fibrils hampers water penetration and the break-down of hydrogen bonds holding the structure together.
To avoid hornification of microfibrillated cellulose (MFC) or nanofibrillated cellulose (NFC), several physicochemical approaches can be used like: (1) supercritical drying, spray drying or freeze drying, (2) use of additives that prevent or reduce hydrogen bonds, (3) rendering MFC/NFC more hydrophobic via chemical modification, or (4) formation of thin webs on paper machine.
In the first category, Turbak et al disclosed a method to produce microfibrillated cellulose where the microfibrillated cellulose was dried by carbon dioxide critical point drying (U.S. Pat. No. 4,374,702 and U.S. Pat. No. 4,378,381). The supercritical drying process is complicated by solvent replacement and the costs are high, with scale up thought to be impractical.
Oven drying, freeze drying, supercritical drying, and spray-drying methods were used to dry microfibrillated or nanofibrillated cellulose suspensions (Vartiainen et al, 2011, Cellulose, 18:775-786 and Peng et al, 2012, Cellulose 19(1): 91-102). Due to hornification of the MFC or NFC, fine and coarse aggregates of MFC or NFC were formed during these drying processes. However, the re-dispersibility of the dried aggregates of MFC or NFC in water was very poor.
In the category of additives, Herrick (U.S. Pat. No. 4,481,076) disclosed a method to produce re-dispersible microfibrillated cellulose using an additive capable of substantially inhibiting hydrogen bonding between the cellulose fibrils. The additive may be sucrose, glycerin, ethylene glycol and propylene glycol, sugar derivatives, starch, inorganic salts such as alkali metal salts of phosphates or borates. Each additive must be used in high amounts, generally between 50 to 100% of the dry weight of MFC. These compounds impair fibrils coalescence during water removal by covering them with a thick layer of water-soluble coating which once put back in water will dissolve to release the fibrils. Properties of never-dried MFC like viscosity can be partially restored with this approach, but the amount of additives needed is impractically high, and adds significantly extra costs to the microfibrillated cellulose products.
Nuopponen et al. (US Pat. No. 0000855 A1) added optical brightening agents (OBAs), such as stilbene, coumarin and pyrazoline compounds, in a process of manufacturing nanofibrillated cellulose pulp to inhibiting hydrogen bonding between cellulose fibrils, which can also create dispersive effect by reducing fibre-water and fibre-fibre bonding that occurs during drying process. It was shown that dried nanofibrillated cellulose pulp containing optical brightening agent dispersed better than the one without optical brightening agent, but the degree of dispersibility of the dried nanofibrillated cellulose pulp containing optical brightening agent was not clear. In addition, optical brightening agents are very expensive additives.
In the approach to render MFC/NFC more hydrophobic via chemical modification, Gardner et al (U.S. Pat. No. 8,372,320 B2) disclosed a drying method of producing dried cellulose nanofibrils comprising atomizing an aqueous suspension of cellulose nanofibrils and introducing the atomized aqueous suspension into a drying chamber of a drying apparatus. The aqueous suspension may include a surface modification agent, such as sodium silicate, fluorosilane, or ethanol, which prevents agglomeration of cellulose nanofibrils by reducing surface tension.
Laukkanen et al (WO2012/107642 A1 & U.S. Pat. 2013/0345416 A1) described a method to produce dried nanofibrillar cellulose by means of organic solvent exchange to remove water, followed by a drying process. Since a large volume of organic solvent is needed, this process to obtain dry nanofibrillar cellulose is not green nor economically viable.
In addition, Bras et al (WO 2014/001699 A1) described a process for manufacturing a fibrillated cellulose powder suitable for being dispersed in an aqueous medium. In this process, monovalent salt (5-20 mmol/l) from the group of sodium chloride, potassium chloride and lithium chloride was added to the fibrillated cellulose suspension and followed by a step of lyophilisation. The fibrillated cellulose suspension was pretreated by enzymatic or chemical such as carboxymethylation.
Eyholzer et al (Cellulose, 17:19-30, 2010) and Cash et al (U.S. Pat. No. 6,602,994 B1) disclosed methods to derivatize the microfibrillated or nanofibrillated cellulose with the introduction of various groups including carboxyl groups. However, the derivatization requires the use of large amounts of the reagent and it has not been established that derivatized MFC can be re-dispersed in water after drying.
A method to produce dry and re-dispersible CF without the need for additives or for the derivatization of cellulose was disclosed (Dorris et al, WO2014/071523 A1) incorporated herein by reference. It involves the formation and drying of a thin web on a fast paper machine. This method requires a paper machine, a very expensive piece of equipment. Although many such machines are idle and available for this purpose, many of these paper machines will eventually be dismantled. Moreover, need to re-dilute the product to form a thin web is an extra step which adds to drying cost.
There is, therefore, a need for developing a cost effective method for drying cellulose nanofilaments or cellulose filaments (CF) without losing their dispersibility in water and hence their superior reinforcement ability in papermaking furnishes, composite materials, or other materials.